This invention concerns devices and method for eliminating multiple-time echoes in Doppler pulse radars which are coherent only on reception.
Multiple-time echoes come from distant objects located beyond the normal radar detection range, and are generated not by the most recent pulse transmitted by the radar, but by one of the preceding pulses. Especially troublesome multiple-time echoes are fixed echoes which come from mountain ranges, since their power is high with respect to that of echoes from useful targets.
It is known that, if equally spaced (in time) radar pulses are coherently transmitted, all echo pulses can be coherently processed, eliminating the problem of fixed multiple-time echoes. Such a transmitter includes an amplifier chain controlled by an oscillator which is extremely stable over durations of several pulse repetition periods, the multiple-time echoes are therefore not received in an uncorrelated condition and can be eliminated by standard cancellation or Doppler fitting, provided they are fixed, in the same way as first-time fixed echoes. However, such coherent transmitters are costly vis-a-vis the magnetron transmitter widely used.
In equally spaced (in time) pulse Doppler radars coherent only on reception (magnetron radars), multiple-time echoes, even if fixed, are no longer eliminated, since such radars operate by storing the phase of the last pulse transmitted by the radar and comparing it to the phases within the received echo pulse train. In that way the Doppler information is recovered, but the phase of pulses transmitted prior to the last pulse is no longer stored once a new pulse occurs. Since the phase of the transmitted pulse varies in a random manner from one repetition period to the next, the multiple-time echoes are uncorrelated and can no longer be eliminated, even though fixed. These echoes occupy the same range gates from one repetition period to the next and are completely mingled with the useful target echoes. The result is a "false alarm" which makes the detection of useful targets very difficult.
One known approach to this problem is based on the difference existing between the distance covered by the multiple-time echoes and the distance covered by the useful target echoes. The multiple-time echoes may cover several kilometers, which represents several range gates of a normal Doppler Radar, while the useful target echoes in most cases occupy only one range gate. A contrast detector compares the signal received in each range gate with the average of the signals received in the time adjacent gates. A useful target is recognized only if the signal in the gate in question sufficiently exceeds this average. Of course, this operation is not limited to range comparisons, but can be used in the same way with respect to bearing. One disadvantage of this method of elimination of multiple-time echoes by contrast detection is that it is ineffective for shorter range multiple-time echoes occupying, for example, in only two or three range gates. The average of the signals of the adjacent gate is, in that case, low and the multiple-time echo signal in the gate in question is much greater, which leads to false recognition of a useful target. Another disadvantage of this method is that it only allows detection of useful targets with an echo power greater than that of the multiple-time echoes. Quite often, in the case of massive mountain ranges, multiple-time echoes are much stronger than the echoes of useful targets.
Another known solution to the problem of elimination of multiple-time echoes in a Doppler radar coherent only on reception consists in using a transmitter with unequally spaced (staggered or randomized PRF) pulses. The transmitted pulses are spaced in a variable manner, which means displacing each pulse by a certain value with respect to its position in a radar with equally spaced pulses. The multiple-time echoes then possess a variable delay with respect to the last pulse transmitted and consequently occupy different range gates from one pulse to the next. Conversely, the first-time echoes have a fixed delay with respect to to the last pulse transmitted and therefore always occupy the same gate. Discrimination is possible by integration of the signals received in the same range gate over several radar repetition periods. This solution is described in particular in U.S. Pat. No. 3,441,930, however, it presents two problems of its own.
In the case of a range gate containing both a first-time echo and a multiple-time echo, the two echoes combine, resulting in erroneous Dopple information, and in the case of a range gate containing no first-time echo, but only a multiple-time echo, the average value resulting from the integration over several periods may exceed the radar detection threshold even though this echo is present only a restricted number of times in the gate, because its power is often very large, and the result is a "false alarm."
The manner in which the present invention deals with the disadvantages of the prior art to provide a novel advantageous solution to the problem will be understood as this description proceeds.